Welcome to the Elza Gate Park provided by the City of Oak Ridge and located at 101 Oak Ridge Turnpike, Oak Ridge, TN. The park commemorates the historical location of the Elza Gate. This was the primary entrance into Oak Ridge in the 1940's when it was a secret city as a part of the Manhattan Project. Here you will find trailheads for Worthington Cemetery Trail and Melton Lake Greenway.
At these coordinates, you will see an interesting rock that includes chert nodules. This EarthCache will try to explain how these chert nodules were formed.
In order to achieve this EarthCache, log your visit and complete the requirements below. When we review your logging task answers, if there is a problem, we will contact you to resolve it. If there is no problem, then your log stands. Any pictures or logs giving away the answers will be deleted.
Send us an email with:
- The text "GC5Z00P Chert Nodules at Elza Gate Park" on the first line.
- How many chert nodules do you count on the northwest face of the host rock? (Do not count the chert along the top of this face. It resembles bedded chert and it is difficult to see individual pieces.)
a. 35 - 39
b. 40 - 44
c. 45 - 49
d. 50 - 55
- Observe the stratification (layering) of the host rock and the chert nodules. Describe the orientation of the layering of (a) the host rock and (b) the chert nodules(for example: horizontal, vertical, x degrees from horizontal).
- Look for a chert tube formation where a nodule is seen on two faces of the host rock. Where on the rock do you see this?
About the location
Oak Ridge lies in the Valley and Ridge Physiographic Province named for the alternating northeast-southwest oriented valleys and ridges. These valleys and ridges were formed at the same time as the Blue Ridge Province (includes the Smoky Mountains) when the supercontinent Gondwana collided with proto-North America (300 - 250 million years ago). As you approach the coordinates you will see outcrops (exposures of bedrock) jutting out of the ground that are all slanted toward the southeast. According to the U. S. Geological Survey, the rock here belongs to the Chickamauga Group from the Ordovician Period (485 - 445 million years ago) and is predominantly limestone.
The host rock is located in a natural part of the woods near the parking lot for Elza Gate Park. While there is no cleared path, it is an easy walk to the area with no need to bushwhack. We have never encountered poisonous plants in the area but you should keep an eye out for them. The same goes for ticks. This photo will give you an idea of what you’ll see at ground zero. (Possible spoiler sections are blurred over.)
What is Chert?
Chert is the general term for very fine-grained and non-porous sedimentary rocks that are composed of silicon dioxide (SiO2). Chert breaks with a conchoidal fracture making it fairly easy to identify. Early people took advantage of how chert breaks leaving sharp edges, and they used it to fashion cutting tools and weapons. The non-scientific equivalent term is flint.
Cherts come in two distinct varieties, nodular chert and bedded chert. The relative importance of nodular chert and bedded chert has changed through geologic time: bedded chert is much more common in the Precambrian Eon, and nodular chert is more common in the Phanerozoic Eon. Cherts are commonly seen, but their total contribution to the sedimentary record is probably not much more than a percent or two.
How are chert nodules formed?
When we think of sedimentary rocks we envision the seafloor covered by layers of shell, coral, algal, and fecal debris. These layers accumulate over time and the pressure from upper layers turns the lower layers into sedimentary rocks. The majority of sedimentary rocks such as limestone and dolostone are formed from these materials. These are carbonate rocks, meaning they are based on carbon compounds (specifically calcium carbonate). There are a few marine organisms that construct their shells or skeletons from silicon compounds. When these organisms die, their skeletons fall to the ocean bottom providing the source material for chert. Diatoms, radiolarians, and the spicules of some sponges provide the silicon-based material.
As the silicon material decomposes it forms silica (silicon dioxide, SiO2). Where there is an abundance of silica it can form a layer and chert beds are formed. These look similar to the carbonate rock layers more commonly seen.
But chert nodules are a completely different story. Previously geologists believed that nodules were formed by direct precipitation of silica debris, as with chert beds, on the ocean bottom. Today the evidence points to a replacement origin. In other words, some silica comes together within the soft sediment that will become the host carbonate rock. This amorphous silica material undergoes a transformation where it starts to crystalize. These crystal seeds then grow and replace the surrounding carbonate material as more silica is brought to the seed through the pores in the surrounding sediment. This explains the variety of shapes of chert nodules from egg-shaped to knobby bodies. There may also be tubes of chert formed as the crystallization follows the path of the feed material. Sometimes the surrounding sediment is incorporated into the chert crystals to produce different colors and types of chert.
Congratulations to BSurprised865 for First to Find!
Special thanks to Billy Pickett of the City of Oak Ridge, TN Recreation & Parks Department for granting permission to place this EarthCache. For more information about the parks and greenways in Oak Ridge please click on this link.
The above information was compiled from the following sources:
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MIT OpenCourseWare: Earth, Atmospheric, and Planetary Sciences Course 12.110, "Sedimentary Geology (Spring 2007)" Chapter 6
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U. S. Geological Survey, “Tennessee Geology”, http://mrdata.usgs.gov/sgmc/tn.html
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Robert G. Maliva and Raymond Siever, "Nodular Chert Formation in Carbonate Rocks", The Journal of Geology Vol. 97, No. 4 (Jul., 1989), pp. 421-433
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http://geology.com/rocks/chert.shtml
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Diatom image: "Diatomeas w" by Damián H. Zanette - Licensed under Public Domain via Wikimedia Commons
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Radiolarian image: "Circogoniaicosahedra ekw". Licensed under Public Domain via Wikimedia Commons
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Sponge spicule image: "Sponge-spicule hg" by Hannes Grobe - Own work. Licensed under CC BY 3.0 via Wikimedia Commons